I have an HDS 12 gen 3, the down imaging has white dots on the screen. Almost look like a fish would but they're spread out and very white in color. Any ideas?
grout-scout's educated guess is a good one. You really have to start eliminating sources to find out if it is interference or not. All the major brands do a decent job of putting filters on their power input, but they can only do so much. If a boat's electrical system is sub-par, issues can be seen.
For instance, do you know if the issue is all the time, or only when the outboard or some other devices are running? Try drifting with EVERYTHING else off and see if the dots are still there. Next, remove both connections for the charger on the starting battery (temporarily as a test, assuming HDS power is from that battery). Warning: do not tinker with battery connections while the outboard is running! I know you are on the water and the charger is not active, but the circuits are still connected.
Do you have a friend with an HDS setup? If your unit isn't flush mounted, could you move it to another boat for a test? Remember, for compatibility it can be Gen 2 or Gen 3, doesn't have to be Touch.